Patent GB-1489756-A (GB)
Title: Optical recording
IPC: G11B7/24085
Filed: 1974-10-14
Abstract: 1489756 Optical recording and playback PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 14 Oct 1974 [17 Oct 1973] 44519/74 Heading G5R A record carrier stores information in an optically readable structure of constant width tracks, each track having first areas (g), Fig. 2 (not shown), alternating along the track with intermediate second areas (t), areas (g) having a different effect on an optical read beam than areas (t) and the information being encoded in the lengths of areas (g) and (t), each track exhibiting substantially spatially periodic lateral excursions of constant zero-peak amplitude (a) and of equal numbers in all tracks, amplitude (a) being smaller than the track width and the number of excursions in each track being substantially smaller than the number of the areas (g) in that track. The transmission or reflection coefficient of areas (g) may differ from that of areas (t) to give an amplitude modulated read beam or areas (g) may be arranged to give a different read beam phase to areas (t) by arranging areas (g) at different heights to areas (t), e.g. by forming #/4 deep pits in a reflecting carrier, # being the read beam wavelength. The average period (p), Fig. 2, of the excursions in a given track is hundreds of times greater than the average length of areas (g) in that track. The excursions may be sinusoidal or triangular, with both track edges in phase. When a video signal is recorded, in order to reduce cross-talk between adjacent tracks the excursions may be provided only at those regions corresponding to the line sync. pulses, with the excursions offset laterally alternately to one side and then the other or all being offset to the same side. A disc playback apparatus, Fig. 4, has a laser 20 giving a linearly polarized beam 21 passing via lenses 22, 24, mirror 23, bi-refringent element 25, such as a Wollaston prism, and #/4 plate 26 to be reflected at record 1 to give a modulated beam which passes to a detector 27 giving an electrical playback signal output Si from high pass filter 29, and a signal Sr from low pass filter 28. Signal Sr is compared in phase with a reference signal derived from the record to produce a tracking control signal for rotating mirror 23. The reference signal indicates the phase of the track excursions actually on the record and may be produced by a separate head reading mechanically, magnetically or optically detectable marks, e.g. on a separate track at the inner or outer circumference of the disc record. If a video signal is recorded, the reference signal can be derived from the line sync. pulses obviating the need for such additional means. A disc recording apparatus, Fig. 5, passes a beam 30 from laser 31 to a photo-resist layer 41 via prisms 32, 33, 38, lens 39, an optical intensity modulator 34 controlled by the information signal to be recorded, and a deflector 44 producing the periodic track excursions. Modulator 34 may comprise an electro-optical crystal which rotates the phase of polarization and an analyser converting such radiation into an intensity variation. The deflector 44 may be an acoustic-optical cell having an optical medium, e.g. glass or water, located between a pair of electro-mechanical transducers producing acoustic waves in said medium giving rise to Bragg diffractions therein so that the beam passing therethrough is partly deflected by an angle proportional to the frequency of the signal supplied to said transducers. The intensity modulator 34 may also alternatively be in the form of an acousto-optical cell, the intensity of the emerging sub-beams, (54), (57), Fig. 6 (not shown), being defined by the amplitude of the signal applied to the electro-mechanical transducers. By feeding such an acousto-optical cell with a signal of variable amplitude and variable frequency a single such cell may be used as both the intensity modulator 34 and the deflector 44.